1. Technical Field
The present disclosure relates to a method for protecting an integrated circuit against attacks aiming to discover sensitive data stored or handled by the integrated circuit. The present disclosure particularly relates to attacks made by means of a light beam such as a laser beam, through the semiconductor substrate on which the circuit is formed.
2. Description of the Related Art
Integrated circuits are vulnerable to attacks on their physical structure, and aiming to discover information they store or handle or to change their operating characteristics. Some attacks involve performing a reading or injecting a signal on an interconnection path, particularly using a laser beam.
The most frequently used means against this type of attack involves using a passive or active shield. Passive shields are generally formed on the circuit's layers of insulator and interconnection paths, that are themselves formed on an active area of a semiconductor substrate. However, such shields can be removed or pierced without the circuit thus protected being able to detect it. On the contrary, active shields are linked to the circuit to enable it to detect when they are removed or pierced. The circuit can then take any appropriate measure to preserve the confidentiality of sensitive information.
For miniaturization reasons in particular, some integrated circuits like those used in video decoders, implement the flip chip technology. This technology involves turning over the substrate of the integrated circuit to place the contact pads formed on the front side of the circuit, directly opposite the contact pads formed on a board such as a printed circuit board. The result is that the backside of the substrate is directly accessible to an attacker wanting to read the content of a memory for example. The protection consisting of forming a passive or active shield on the interconnection layers is thus inappropriate as the shield would then be opposite the printed circuit board and not on the backside of the substrate on which the integrated circuit is formed.
FIG. 1 schematically represents an integrated circuit CI in cross-section. The integrated circuit CI comprises an active area AL formed on a substrate SUB made of a semiconductor material, and layers of interconnection paths ML formed on the active area, alternated with insulating layers. The integrated circuit CI also comprises on its front side connection balls SD deposited on contact pads, in electrical contact with interconnection paths of the integrated circuit. To connect it to other circuits (not represented), the integrated circuit CI is turned over so as to weld the connection balls SD onto contact pads formed on a front side of a larger board SB1, for example a printed circuit board (PCB). Interconnection paths ITC are also formed in one or more layers on the board SB1. The backside of the board SB1 may comprise contact pads on which connection balls BP are deposited.
An attack of the integrated circuit CI thus connected onto the board SB1 can involve thinning down, by grinding, the circuit from the backside, i.e., the inactive side of the substrate, until the active area AL is reached. To show an attack of this type, FIG. 2 schematically represents a cross-section of the circuit CI, after the thinning step. This grinding step can reduce the thickness of the circuit CI from about 500 μm to a few tens of micrometers. This thinning step aims to free the active area AL so as to make it directly accessible by a laser beam LB. A laser beam can indeed be used to read by photo-electric effect the content of a memory or of registers formed in the area AL or to detect signals sent by interconnection lines.